Conventionally, a cleaning device for cleaning residual toner off an image carrier such as an intermediate transfer belt generally employs a method for putting a blade made of rubber in contact with a surface of the image carrier to mechanically scrape the toner off the surface, or a method for putting a bias-applied roll brush in contact with the image carrier to electrically attract the toner.
In the method for putting the bias-applied roll brush in contact with the image carrier, cleaning is performed by electrically attracting toner to the roll brush. Therefore, the toner with a polarity opposite to that of the bias applied to the roll brush is cleaned. The toner remaining on the image carrier and subjected to cleaning sometimes has a polarity charged opposite to the original polarity of the toner due to the influence of the bias (electric field) which is applied for transfer of the toner onto paper sheets or an intermediate transfer body.
This tendency is more notable in toner remaining on an intermediate transfer body when toner is transferred from the intermediate transfer body such as an intermediate transfer belt to the paper sheet than in toner remaining on a photoconductor when the toner is transferred from the photoconductor to the paper sheet. This is caused by the following reasons. That is, toner has one layer in the case of transferring the toner from the photoconductor to the paper sheet and the intermediate transfer body. On the other hand, toner has a mixture of one to four layers in the case of transferring the toner from the intermediate transfer body, e.g., an intermediate transfer belt, to the paper sheet because toner layers are superposed on top of each other on the belt. A transfer bias applied for transferring the toner including the four layer toner from the intermediate transfer body is higher than that for transferring the one layer toner from the photoconductor, and therefore, a part of one layer toner is easily influenced by this high transfer bias.
Thus, when the residual toner is cleaned with the bias-applied roll, the roll brush is not used independently, but two roll brushes which are made of an identical material are placed side by side in the rotation direction of the image carrier, as seen in cleaner devices or cleaning devices disclosed in JP H10-10942 A, JP 2002-229344 A and JP 2002-207403 A, for example. The cleaner devices or the cleaning devices further includes a toner collection roller and a scraper downstream of the roll brush, wherein the toner collection roller is for collecting the toner taken into the roll brush with use of a potential difference, and wherein the scraper is for mechanically scraping off the toner collected on the toner collection roller. In the cleaner device and the cleaning device, two biases with polarities different from each other are respectively applied to two roll brushes placed side by side, so that each of the roll brushes collects toner charged to a polarity opposite to the applied polarity.
However, there is a following problem in the conventional cleaning device using two bias-applied roll brushes.
That is, the roll brush is influenced not only by applied bias but also by electric charge caused by contacting or rubbing with toner as described below.
In triboelectric charging caused by contacting or rubbing between two substances, generally, polarities of the two substances i.e. negative and positive polarities determined by combinations of the two contacting or rubbing substances. Their polarities can be known from a charge ranking list (charging array) shown in FIG. 7. Two substances which come into contact or rub are more highly charged when their physical positions are further away from each other on the charge ranking list, whereas the two substances are not highly charged when their physical positions are close to each other.
However, the charge ranking list is not absolute but may have some changes because the triboelectric charge also depends on the surface state of materials or other environments. Base material of the toner is styrene acrylics. Since other materials such as external additive are added against the toner, the position of the toner is presumably closer to neutrality (i.e. the center) than the position of styrene acrylics on the charge ranking list shown in FIG. 7.
In brush-cleaning with use of the bias-applied roll brush, a brush fiber which constitutes the roll brush is influenced by triboelectric charges of both the toner and the intermediate transfer belt since the brush fiber has contact with both of them. However, the triboelectric charge between the brush fiber and the toner is dominant over the triboelectric charge between the brush fiber and the intermediate transfer belt because the roll brush electrically attracts the charged toner to the brush fiber so as to collect the toner.
Description is now given on the case where cleaning is performed by, for example, attracting negatively charged toner 1 to a brush fiber 2 to which a positive voltage has been applied, as shown in FIG. 8. In this case, material to be triboelectrically charged to a positive polarity against the toner 1 is used as material of the brush fiber 2. Then, the rubbing between the toner 1 and the brush fiber 2 causes the surface of the brush fiber 2 to be charged to a positive polarity and the toner 1 to be charged to a negative polarity. Thus, rubbing with the brush fiber injects the negative charge into the negatively charged toner 1, which toner is the target of cleaning. As the result, the negatively charged toner 1 is charged to be more negative. Consequently, a larger potential difference (or electric field) is generated between the toner 1 and the brush fiber 2 to which the positive voltage has been applied. Thereby, cleaning of the toner 1 is facilitated. It should be noted that in FIG. 8, a minus sign illustrated by a large letter on the central portion of the toner 1 expresses an original negative charge polarity, whereas other minus signs illustrated by a small letter express negative triboelectric charge polarity. Plus signs illustrated with a small letter in the brush fiber 2 also express positive triboelectric charge polarity.
Similarly, in the case where cleaning is performed by attracting a positively charged toner 3 to a brush fiber 4 to which a negative voltage has been applied, as shown in FIG. 9, material to be triboelectrically charged to a negative polarity against the toner 3 is used as material of the brush fiber 4. The rubbing between the toner 3 and the brush fiber 4 causes the surface of the brush fiber 4 to be charged to a negative polarity and the toner 3 to be charged to a positive polarity. Thus, rubbing with the brush fiber 4 injects the positive charge into the positively charged toner 3, which toner is the target of cleaning. As the result, the positively charged toner 1 is charged to be more positive. Consequently, a larger potential difference (or electric field) is generated between the toner 3 and the brush fiber 4 to which the negative voltage has been applied. Thereby, cleaning of the toner 3 is facilitated.
Thus, cleaning performance is enhanced by arranging that the polarity of the bias applied to the brush fibers 2 and 4 should be identical to the triboelectric charge polarity of the brush fibers 2 and 4 against the toner 1 and 3, respectively.
In the conventional cleaner device and cleaning device using two roll brushes made of identical material, as in the cases of the cleaner device and the cleaning device disclosed in JP H10-10942 A, JP 2002-229344 A and JP 2002-207403 A, biases having different polarities to each other are respectively applied to the brush fibers of two roll brushes made of an identical material, so as to collect toners having polarities opposite to the applied polarities.
Therefore, in one of the roll brushes (hereinafter referred to as a first roll brush), a polarity of the bias applied to the brush fiber is identical to a triboelectric charge polarity of the brush fiber against the toner, as shown in FIG. 8 and FIG. 9. As the result, a larger potential difference (or electric field) is generated between the toner and the brush fiber, which facilitates cleaning of the toner.
On the other hand, in the other of the roll brushes (hereinafter referred to as a second roll brush), a polarity of the bias applied to the brush fiber is different from a triboelectric charge polarity of the brush fiber against the toner.
Specifically, as shown in FIG. 10, in the case where cleaning is performed by attracting a positively charged toner 1 to a brush fiber 2 to which a negative voltage has been applied, and where material triboelectrically charged to the positive polarity against the toner 1 is used as material of the brush fiber 2, rubbing between the toner 1 and brush fiber 2 causes the surface of the brush fiber 2 to be charged to the positive polarity and the toner 1 to be charged to the negative polarity. Thus, rubbing with the brush fiber 2 causes the negative charge to be injected into the positively charged toner 1, which toner is the target of cleaning, to neutralize the positively charged toner 1. This decreases the potential difference (or electric field) between the toner 1 and the brush fiber 2 to which the negative voltage has been applied. Therefore, cleaning of the toner 1 becomes difficult and failure of cleaning may easily occur. When a large amount of negative charge is injected into the positively charged toner 1, the positively charged toner completely changes to negatively charged toner, and then, the toner remains on the intermediate transfer belt 5 without being cleaned. That is, the failure of cleaning occurs.
Similarly, as shown in FIG. 11, in the case where cleaning is performed by attracting negatively charged toner 3 to a brush fiber 4 to which a positive voltage has been applied, and where material triboelectrically charged to a negative polarity against the toner 3 is used as material of the brush fiber 4, the similar phenomenon to the above occurs. That is, rubbing with the brush fiber 4 causes the positive charge to be injected into the negatively charged toner 3, which toner is the target of cleaning, to neutralize the charge of the toner 3. Therefore, cleaning performance is degraded.
Thus, cleaning performance is degraded when a polarity of the bias applied to the brush fibers 2, 4 is different from a triboelectric charge polarity of the brush fibers 2, 4 against the toner 1, 2, respectively.
As is clear from the foregoing, in the cleaner device and cleaning device disclosed in JP H10-10942 A, JP 2002-229344 A and JP 2002-207403 A, cleaning performance is deteriorated with the second roll brush in which a polarity of the bias applied to the brush fiber is different from a triboelectric charge polarity of the brush fiber. As a result, the cleaning performance is totally deteriorated since the facilitated cleaning performance of the first roll brush is offset by the degraded cleaning performance of the second roll brush.